Alga is a flora mainly composed of photosynthetic plants which grow in waters such as lake water, swamp water, river water, and ocean water. It also lives in soil or attaches to live on rocks, tree surfaces, body surfaces of animals, and the like besides living under water. Algae are generally classified by color into green algae, brown algae, and red algae, or by characteristics. They may be classified into macroalgae and microalgae as well. Algae have been deeply concerned with mankind from ancient times, and used as beneficial living aquatic resources. However, they also behave unbeneficially when culturing fish or the like in a fishing net or a net-cage by adhering to and growing on a fish net with the result that streams are prevented and fish suffers from oxygen shortage, and when raising aquatic animals under closed environments such as in a fish tank or a culture pond, proliferous microalgae grow to cause pollution of water for raising aquatic animals or diseases of them. In addition, the algae which emerge and grow on the draft of seacrafts can prevent smooth ship navigation.
When raising fish for appreciation in an aquarium or at home, the effect of algae is not only pollution of water for raising aquatic animals as described but also difficulty in removing algae attached and grown on the glass of a fish tank and spoiled appearance of the tank.
When culturing aquatic animals for studying purposes, the growth of microalgae prevents normal growth of aquatic animals being cultured, which in turn markedly affects the results of the study.
Therefore, various algaecides have been studied for inhibiting or preventing emergence and growth of algae, and quaternary ammonium salts such as benzalkonium chloride and chlorine compounds such as sodium hypochlorite have been mainly used. Yet, they are not desirable since they are highly toxic and harmfully affect aquatic animals.
Incidentally, germanium which interrupts intake of silicon into cell membranes of diatom to inhibit growth shows a preventive effect on the growth of some particularly proliferous diatom from among algae [Lewin, J., Phycologia, 6, 1-12 (1966), and others], and germanium compounds have been starting to be employed. While germanium proves effective in preventing or inhibiting the growth of certain diatom, it fails to prevent growth of diatom having resistance to toxicity by germanium, green algae, red algae, and others.
There has also been made an attempt to use herbicides as algaecides, but none has proved usable yet for the reason that they are highly toxic to aquatic animals. Diuron is generally used as a herbicide for its inhibitory action on photosynthesis of plants to inhibit the growth of the plants. The minimum inhibitory concentration (MIC) of diuron against growth of microalgae and Cyanophyceae is reportedly from 1.1 to 2.8 .mu.g/ml (1.1-2.8 ppm ) [D. M. Paterson et al, Lett. Appl. Microbiol., 7, 87-90 (1988)]. From among various herbicides, Call, D. J. et al [Arch. Environ. Contam. Toxi., 16 (5), 607-613 (1987)] examined acute toxicity of diuron against fathead minnow, and reported that the amount to kill 50% of the fathead minnow (LD.sub.50) was 14.2 mg/L (14.2 ppm) at 96 hours and 7.7 mg/L (7.7 ppm) at 192 hours, and that the concentration which did not exert bad influence on juvenile fish and eggs thereof was 33.4 .mu.g/L (0.0334 ppm). This reference has also reported acute toxicity of other known aquatic animals [e.g. LD.sub.50 (96 hours) of larval striped bass, 0.5 mg/L] and the data therein show rather great diversity concerning sensitivity to diuron among various species of aquatic animals. These prior art references indicate that the MIC of diuron used for algae my display high toxicity depending on the kind and growth stages of aquatic animals, and in view of this, diuron cannot be used for removing and destroying algae disregarding the animals living under water. Accordingly, it has been substantially impossible to use herbicides such as diuron in water for raising aquatic animals for the purpose of inhibiting or preventing emergence and growth of algae without causing any influence on aquatic animals.
Meanwhile, artificial seawater having a composition similar to that of natural seawater has been increasingly used in recent years in place of natural seawater for raising and culturing living organisms in seawater, or studying ecology of them. Even if algae do not exist in artificial seawater at the time of its preparation, algae attached or commensal to the marine animal to be raised in the artificial seawater could grow. The artificial seawater, therefore, cannot always inhibit or prevent emergence of algae.